線程
線程的幾種狀態
public enum State {
//創建後尚未啓動的線程處於這種狀態。
NEW,
//Runable包括了操作系統現線程狀態中的Runing和Ready,也就是處於次狀態的線程有可能正在執行,也有可能正在等待着CPU爲它分配執行時間。
RUNNABLE,
//線程被阻塞了,“阻塞狀態”與“等待狀態”的區別是:“阻塞狀態”在等待的時候有一個排它鎖,這個事件將在另外一個線程放棄這個鎖的時候發生;“等待狀態”則是等待一段世間,或者喚醒動作的發生。在程序等待進入同步區域的時候,線程進入這種狀態。
BLOCKED,
//處於這種狀態的線程不會被分配CPU執行時間,它們要等待被其他線程顯式的喚醒。
WAITING,
//處於這種狀態的縣城也不會被分配CPU執行時間,不過無需等待被其它線程顯式的喚醒,在一定時間之後它們會由系統自動喚醒,在一定時間之後它們會由系統自動喚醒。
TIMED_WAITING,
//已終止線程的線程狀態,線程已經結束執行。
TERMINATED;
}
實現線程的幾種方式(4種)
1)繼承Thread類,實現run方法
public class MyThread extends Thread {
@Override
public void run() {
for (int i = 0; i < 10; i++) {
System.out.println(i);
}
}
}
MyThread myThread = new MyThread();
myThread.start();
System.out.println("------------");
2)實現Runnable接口,實現run方法
public class MyRunable implements Runnable {
@Override
public void run() {
for (int i = 0; i < 10; i++) {
System.out.println(Thread.currentThread().getName()+"----"+i);
}
}
}
Thread thread = new Thread(new MyRunable());
thread.start();
System.out.println("------------");
3)實現Callable接口,實現call方法
和上面的方式相比,這種有返回結果
public class MyCallable implements Callable<Integer> {
@Override
public Integer call() throws Exception {
int sum = 0;
for (int i = 0; i < 10; i++) {
sum += i;
System.out.println(Thread.currentThread().getName() + "----" + i);
}
return sum;
}
}
MyCallable myCallable = new MyCallable();
FutureTask<Integer> futureTask = new FutureTask(myCallable);
Thread thread = new Thread(futureTask);
thread.start();
System.out.println("---------------");
Integer result = futureTask.get();//get方法是阻塞方法,只有當自定義的線程運行完纔會得到結果
System.out.println("result: " + result);
4)線程池創建線程
//創建線程池
ExecutorService pool = Executors.newFixedThreadPool(5);
//爲線程池中的線程分配任務
MyCallable myCallable = new MyCallable();
Future<Integer> result = pool.submit(myCallable);
//關閉線程池
pool.shutdown();
System.out.println(result.get());
線程池
線程池工作原理
1 先向核心線程 提交任務
2 如果核心線程滿了 把任務放在隊列中
3 如果隊列也滿了 ,那就擴招 非核心線程
4 最大線程 和 任務隊列都滿了,就執行拒絕策略
線程池的核心參數
以下面爲例
ExecutorService executorService = Executors.newFixedThreadPool(5);//一個池子有5個線程
跟一下newFixedThreadPool方法
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
繼續跟
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), defaultHandler);
}
繼續跟
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler) {
if (corePoolSize < 0 ||
maximumPoolSize <= 0 ||
maximumPoolSize < corePoolSize ||
keepAliveTime < 0)
throw new IllegalArgumentException();
if (workQueue == null || threadFactory == null || handler == null)
throw new NullPointerException();
this.corePoolSize = corePoolSize;
this.maximumPoolSize = maximumPoolSize;
this.workQueue = workQueue;
this.keepAliveTime = unit.toNanos(keepAliveTime);
this.threadFactory = threadFactory;
this.handler = handler;
}
此時ThreadPoolExecuter的7個參數就出來了
1)int corePoolSize
線程池中常駐的核心線程數
2)int maximumPoolSize
線程池中允許同時容納執行的最大線程數,此值必須大於等於1
3)long keepAliveTime
多餘的空閒線程的存活時間,當前池中線程數量超過corePoolSize時,當空閒時間達到keepAliveTime時,多餘線程會被銷燬直到只剩下corePoolSize
4)TimeUnit unit
keepAliveTime的單位
5)BlockingQueue<Runnable> workQueue
任務隊列,被提交但尚未被執行的任務
6)ThreadFactory threadFactory
表示生成線程池中工作線程的線程工程,用於創建線程,一般默認即可
7)RejectedExecutionHandler handler
拒絕策略,表示當隊列滿了,並且工作線程大於等於線程池的最大線程數(maximumPoolSize )時如何來拒絕請求執行的runable的策略
自定義線程池的原因
自定義線程池
代碼
ExecutorService executorService = new ThreadPoolExecutor(2, 5, 2L,
TimeUnit.SECONDS,
new LinkedBlockingQueue<Runnable>(3), Executors.defaultThreadFactory(),
new ThreadPoolExecutor.AbortPolicy());
其中
corePoolSize=2
maximumPoolSize=5
keepAliveTime=2
TimeUnit=秒
workQueue=new LinkedBlockingQueue<Runnable>(3) ,裏面的構造方法傳入3,否則默認爲Integer.max(源碼)
threadFactory=Executors.defaultThreadFactory() 一般就用Executors的默認的線程工廠
handler=new ThreadPoolExecutor.AbortPolicy() 這個是線程池默認的拒絕策略
爲什麼這麼配?
corePoolSize=2 maximumPoolSize=5 這兩個數怎來的?
如果是CPU密集型,maximumPoolSize=CPU核數+1
CPU核數怎麼看:
System.out.println(Runtime.getRuntime().availableProcessors());
如果是IO密集型,maximumPoolSize=2*CPU (這個不太準備,自行百度)
參考:
什麼是CPU密集型、IO密集型?
https://blog.csdn.net/youanyyou/article/details/78990156
拒絕策略
- 1) new ThreadPoolExecutor.AbortPolicy()(默認)
---->這種拒絕策略當達到maximumPoolSize+隊列最大值後就會中斷報異常
ExecutorService executorService = new ThreadPoolExecutor(2, 5, 2L,
TimeUnit.SECONDS,
new LinkedBlockingQueue<Runnable>(3), Executors.defaultThreadFactory(),
new ThreadPoolExecutor.AbortPolicy());
此定義的線程爲最大線程數爲5,阻塞隊列爲3,也就是最大運行同時提交8個線程,如果我同時提交8個線程,那沒有問題,運行5個,3個在隊列中,沒有問題
如果我現在同時運行9個,那就會觸發拒絕策略,測試代碼如下
public class Start {
public static void main(String[] args) {
//自定義線程池,最大線程數爲5,等待隊列最大爲3,最大運行同時提交最大線程數爲5+3=8
ExecutorService executorService = new ThreadPoolExecutor(2, 5, 2L,
TimeUnit.SECONDS,
new LinkedBlockingQueue<Runnable>(3), Executors.defaultThreadFactory(),
new ThreadPoolExecutor.AbortPolicy());
try {
for (int i = 0; i < 9; i++) {
int finalI = i;
executorService.execute(() -> {
try {
TimeUnit.SECONDS.sleep(3);//睡3秒,才能達到併發目的
} catch (Exception e) {
e.printStackTrace();
} finally {
}
System.out.println(Thread.currentThread().getName() + "\t" + finalI);
});
}
} catch (Exception e) {
e.printStackTrace();
} finally {
executorService.shutdown();
}
}
}
- 2)new ThreadPoolExecutor.CallerRunsPolicy()
----> 該策略既不會拋拋棄任務,也不會拋出異常,而是將某些任務退到調用者,從而降低新任務的流量。
例如下面的測試代碼,線程池最大運行8個(5個運行,3個等待),第九個就回退給main方法區運行
public static void main(String[] args) {
//自定義線程池,最大線程數爲5,等待隊列最大爲3,最大運行同時提交最大線程數爲5+3=8
ExecutorService executorService = new ThreadPoolExecutor(2, 5, 2L,
TimeUnit.SECONDS,
new LinkedBlockingQueue<Runnable>(3), Executors.defaultThreadFactory(),
new ThreadPoolExecutor.CallerRunsPolicy());
try {
for (int i = 0; i < 9; i++) {
int finalI = i;
executorService.execute(() -> {
try {
TimeUnit.SECONDS.sleep(3);//睡3秒,才能達到併發目的
} catch (Exception e) {
e.printStackTrace();
} finally {
}
System.out.println(Thread.currentThread().getName() + "\t" + finalI);
});
}
} catch (Exception e) {
e.printStackTrace();
} finally {
executorService.shutdown();
}
}
}
- 3)new ThreadPoolExecutor.DiscardPolicy()
---->該策略默默的丟棄無法處理的任務,不予任何處理也不拋棄異常。如果運行任務丟失,這是最好的一種策略。
例如下面測試代碼,丟棄了第9個任務
public class Start {
public static void main(String[] args) {
//自定義線程池,最大線程數爲5,等待隊列最大爲3,最大運行同時提交最大線程數爲5+3=8
ExecutorService executorService = new ThreadPoolExecutor(2, 5, 2L,
TimeUnit.SECONDS,
new LinkedBlockingQueue<Runnable>(3), Executors.defaultThreadFactory(),
new ThreadPoolExecutor.DiscardPolicy());
try {
for (int i = 0; i < 9; i++) {
int finalI = i;
executorService.execute(() -> {
try {
TimeUnit.SECONDS.sleep(3);//睡3秒,才能達到併發目的
} catch (Exception e) {
e.printStackTrace();
} finally {
}
System.out.println(Thread.currentThread().getName() + "\t" + finalI);
});
}
} catch (Exception e) {
e.printStackTrace();
} finally {
executorService.shutdown();
}
}
}
- 4)new ThreadPoolExecutor.DiscardOldestPolicy()
---->拋棄隊列中等待最久的任務,然後把當前任務加入到隊列中嘗試再次提交當前任務(換句話說:長江後浪推前浪,把前浪拍死在沙灘上)
類似上面new ThreadPoolExecutor.DiscardPolicy()